[SOLVED]: Same Web Property ID Is Tracked Twice | Google Tag Manager | Magento 2

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If your Tag Assistant is firing this warning 'Same web property ID is tracked twice' while testing in your Magento 2 site, then it's because you've enabled Google Analytic in your Magento store backend, and also in Google Tag Manager.

SOLUTION: Enable Google Analytics tracking either from your site's backend or by using Google Tag Manager, but not both.

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If you want to use Google Tag Manager for Google Analytics then disable tracking from your site's backend.